Laser light beam generating apparatus using an electromagnetic actuator for reflector positioning

ABSTRACT

A laser light beam generating apparatus includes at least one light beam source, first and second reflectors, a non-linear optical crystal element and an actuator. The light beam source emits a light beam. The non-linear optical crystal element is provided between the first reflector and the second reflector. A light beam emitted from the light beam source is incident on the non-linear optical crystal element through the first reflector. The actuator actuates at least one of the first and second reflectors along an optical axis of the light beam emitted from the light beam source.

This is a divisional of application Ser. No. 08/024,627, filed Mar. 1,1993 U.S. Pat. No. 5,367,537.

BACKGROUND

1. Field of the Invention

This invention relates to a laser light beam generating apparatus. Moreparticularly, the present of invention relates to a laser light beamgenerating apparatus in which a laser light beam converted wavelength isgenerated by a non-linear optical crystal element.

2. Background of the Invention

It has hitherto been proposed to realize wavelength conversion by takingadvantage of the high power density within a resonator. For example,second harmonic generation (SHG) is often achieved by placing anon-linear optical crystal in an external resonator in an attempt toimprove the efficiency of the wavelength conversion.

As an SHG used the non-linear optical crystal element providing theresonator, the resonator which includes at least a pair of mirrors, alaser medium and a non-linear optical crystal element is known. In thisresonator, the laser medium and the non-linear optical crystal elementare provided between the pair of mirrors. With this type of the laserlight beam generating apparatus, the second harmonic laser light beam istaken out efficiently by phase matching the second harmonic laser lightbeam with respect to the laser light beam of the fundamental wavelengthby a non-linear optical crystal element provided within the resonator.

There is also known an external resonant method according to which alaser light beam from a laser light source is introduced into anexternal resonator as laser light beam of a fundamental wavelength andpropagated through a non-linear optical crystal element back and forthfor a resonant operation to generate a second harmonic laser light beam.In the external resonant method, the finesse value of the resonator,corresponding to a Q-value of resonation, is set to a larger value ofabout 100 to 1000 to set the light density within the resonator to avalue hundreds of times as large as the incident light density. As aresult, this type resonator can take advantage effectively of non-lineareffects of the non-linear optical crystal element within the resonator.

Meanwhile, for producing laser light beams of second or higher harmonicsor so-called sum frequency according to the external resonant method, itis necessary to realize extremely fine position control of limitingchanges or errors of the optical path length of the resonator to lessthan 1/1000 or 1/10000 of the wavelength, that is less than 1 Å.

The conventional practice in limiting the resonator length has been tohave the reflective mirrors of the resonator supported by stackedpiezoelectric elements by so-called PZT and to feed an error signalproportional to changes in the resonator length back to the stackedpiezoelectric elements to complete a servo loop for automaticallycontrolling and stabilizing the resonator length.

In general, piezoelectric elements have multiple resonance at intervalsof several to tens of kilohertz frequencies and have phase delay overthe entire frequency range due to self capacitance. As a result, it is.difficult to spread frequency range of the servo range to severalkilohertz. Since the stacked piezoelectric elements are in need of ahigh driving voltage of hundreds to thousands of volts the drivingelectric circuit is complicated and expensive.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a laserlight beam generating apparatus to resolve above-described problem.

It is an another object of the present invention to provide a laserlight beam generating apparatus to improve a control operation oflimiting changes or errors of the optical path length of the resonator.

According to a first embodiment of the present invention, there isprovided a laser light beam generating apparatus including at least onelight beam source, a first reflector, a second reflector, a non-linearoptical crystal element and an actuator. The light beam source emits alight beam. The non-linear optical crystal element is provided betweenthe first and second reflectors. A light beam is incident on thenon-linear optical crystal element through the first reflector. Theactuator actuates at least one of the first and second reflectors alongan optical axis of the light beam emitted from the light beam source.

According to a second embodiment of the present invention, there isprovided a laser light beam generating apparatus having at least onelight beam source, a first resonator, a second resonator and anactuator. The first resonator includes first and second reflectors and alaser medium into which the pumping light beam is incident from thelight beam source through the first reflector. The second resonatorincludes third and fourth reflectors and a non-linear optical crystalelement in which the light beam from the first resonator is incidentthrough the third reflector. The actuator actuates at least one of thefirst, second, third and fourth reflectors along an optical axis of thelight emitted from the first resonator.

Since an electromagnetic actuator is employed as a driver forcontrolling the optical path length of the resonator with high accuracy,the servo range may be increased to tens of kilohertz to permit stablecontrol and highly efficient wavelength conversion. Since a low drivingcurrent for the electromagnetic actuator suffices, it becomes possibleto simplify the circuitry and to reduce production costs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be more readily understood with reference to theaccompanying drawing, wherein:

FIG. 1 is a schematic block diagram showing an embodiment of a laserlight generating apparatus according to the present invention.

FIG. 2 is a graph showing changes in the power reflection with respectto the optical path phase difference of a resonator employed in theembodiment shown in FIG. 1.

FIG. 3 is a graph showing changes in the phase of reflection withrespect to the optical path phase difference of the resonator employedin the embodiment shown in FIG. 1.

FIG. 4 is a waveform diagram showing detection signals of the reflectedlight beam from the resonator.

FIG. 5 is a waveform diagram showing a power component of the reflectedlight beam detection signals.

FIG. 6 is a Waveform diagram showing modulated signal component of thereflected light beam detection signals.

FIG. 7 is a waveform diagram showing a sin(.sup.ω_(m) t) of themodulated signal component of the reflected light beam detectionsignals.

FIG. 8 is a waveform diagram showing a cos(.sup.ω_(m) t) of themodulated signal component of the reflected light beam detectionsignals.

FIG. 9 is a perspective view showing a concrete example of anelectromagnetic actuator, with portions thereof being broken away.

FIG. 10 is a schematic perspective view showing a spiral spring plateemployed in the electromagnetic actuator shown in FIG. 9.

FIG. 11 is a Bode diagram showing the gain of transmissioncharacteristics of the actuator shown in FIG. 9.

FIG. 12 is a Bode diagram showing the phase of transmissioncharacteristics of the actuator shown in FIG. 9.

FIG. 13 is a block diagram showing a schematic arrangement of a servocontrol system.

FIG. 14 is a block diagram showing a schematic arrangement of thecircuitry for detecting error signals in the optical path length of theresonator.

FIG. 15 is a Bode diagram showing frequency characteristics of a closedloop servo system.

FIG. 16 is a waveform diagram showing error signals and reflected lightbeam detection signals when the reflecting surface of the resonator isdeviated along the optical axis without servo control.

FIG. 17 is a waveform diagram showing error signals and reflected lightbeam detection signals under a servo control operation.

FIG. 18 is a schematic block diagram showing another embodiment of thelaser light beam generating apparatus according to the presentinvention.

FIG. 19 is a schematic block diagram showing an example of a first basicarrangement of the laser light beam generating apparatus according tothe present invention.

FIG. 20 is a schematic block diagram showing an example of a secondbasic arrangement of the laser light beam generating apparatus accordingto the present invention.

FIG. 21 is a schematic block diagram showing an example of a third basicarrangement of the laser light beam generating apparatus according tothe present invention.

FIG. 22 is a schematic block diagram showing an example of a fourthbasic arrangement of the laser light beam generating apparatus accordingto the present invention.

FIG. 23 is a schematic block diagram showing an example of a fifth basicarrangement of the laser light beam generating apparatus according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, in a schematic block diagram of an embodiment of the laserlight beam generating apparatus according to the present invention.

Referring to FIG. 1, a laser light beam of a fundamental wavelength isemitted from a laser light source 11, such as a semiconductor laserdevice, e.g. a laser diode, or a second harmonic generating (SHG) laserlight source device. The laser light beam of the fundamental wavelengthis phase modulated by a phase modulator 12 employing an electro-optical(EO) device or an acoustic-optical (AO) device before being incident onan external resonator 15 via an optical element 13 for detecting thereflected light beam from the resonator 15 and a light converging lens14. The external resonator 15 is made up of a reflecting surface 16 of aconcave mirror, a reflecting surface 17 of a plane mirror and anon-linear optical crystal element 18 interposed therebetween. The stateof resonance is produced when the optical path length L_(R) between thereflecting surfaces 16, 17 of the resonator 15 is a preset length andthe optical path phase difference Δ is an integer number times 2π withthe reflection and the phase of reflection being acutely changed nearthe resonance phase. At least one of the reflective surfaces 16, 17 ofthe resonator 15, for example, the reflective surface 17, is adapted forbeing driven along the optical axis by electromagnetic actuator 19.

If an SHG laser light source device is used as the laser light source 11for generating a single-mode laser light beam of the wavelength of 532nm which is supplied to the external resonator 15, the non-linearoptical crystal element 18 of barium borate (BBO) is used in theresonator 15 and, by taking advantage of its non-linear optical effects,a laser light beam of the wavelength of 266 nm, which is the secondharmonic wave of the input laser light beam of 532 nm (or the fourthharmonic wave if the input light beam is the SHG laser light beam) isgenerated. The reflective surface 16 of the concave mirror of theexternal resonator 15 is a dichroic mirror which reflects substantiallyall of the input light beam of the wavelength of 532 nm, while thereflective surface 17 of the plane mirror is a dichroic mirrorreflecting substantially all of the input light beam and transmittingsubstantially all of the output light beam having the wavelength of 266nm.

An oscillator 21 outputs a modulating signal with e.g. a frequency fm=10MHz for driving the optical phase modulator 12 to phase modulator 12 viadriver 22. The reflected or return laser light beam transmitted toresonator 15 is detected via reflective surface 13 and a photodetector23, such as a photodiode. The reflected light beam detection signal istransmitted to a synchronous detection circuit 24. Modulating signalsfrom oscillator 21 are supplied, if necessary, after waveshaping, phasedelaying, etc. to the synchronous detection circuit 24, and multipliedby the reflected light detection signal, for synchronous detection.Detected output signals from the synchronous detection circuit 24 aresupplied via a low-pass filter (LPF) 25, an output of which is aresonator optical length error signal as later explained. This errorsignal is transmitted to a driver 26, a driving output signal of whichactuates the actuator 19 for shifting the reflective surface 17 alongthe optical axis by way of a servo control for reducing the error signalto zero. In this manner, the optical path length L_(R) of the externalresonator 12 is controlled to be a length corresponding to a localminimum of a reflection curve (resonant point).

The electromagnetic actuator 19 may be a so-called voice coil drivingtype actuator and the double resonance frequency can be rendered equalto tens of KHz to 100 KHz or higher. As the servo loop resonancefrequency is raised, and phase deviations are minimized, the servo range(cut-off frequency) can be increased to e.g. 20 KHz or tens of KHz.Since a low driving current of tens to hundreds of milliamperes sufficesfor driving the electromagnetic actuator 19, the driving electriccircuit may be simplified and rendered inexpensive. Consequently, itbecomes possible to provide, in a method for effectively utilizingnonlinear effects employing the external laser resonator method, aninexpensive system for stably suppressing changes in the resonatorlength to less than 1/1000 to 1/10000 of a wavelength, that is to lessthan 1 Å.

The principle of introduction of a laser light into the externalresonator 15, or a so-called Fabry-Perot resonator, and error detection,is explained. Such a resonator is brought into a resonant state when theoptical path phase difference Δ is equal to an integer number times 2πwith the reflection phase being acutely changed near the resonant phase.Frequency control of the resonator by taking advantage of the phasechanges is disclosed for example in "Laser Phase and FrequencyStabilization Using an Optical Resonator" by R. W. P. Drever et al.,Applied Physics-B 31.97-105 (1983). The principle of detection of theerror signal by this technique is hereinafter explained.

In general, if a non-linear optical element having refractive index nand a thickness L is present within a Fabry-Perot resonator, the opticalpath phase difference Δ is 4πnL/λ. If the single-pass transmittance isT, the single-pass SHG conversion efficiency is η, the reflectance atthe incident surface is R1 and the reflection at the outgoing surface isR₂, the complex reflection r becomes ##EQU1## where Rm=R₂ (T (1-η))².The absolute value of r (power reflection) and the phase (reflectionphase) are shown in FIGS. 2 and 3, respectively. By taking advantage ofthese phase changes, the values of the resonant frequency of of theexternal resonator 15 and the frequency fc of the fundamental wavelengthlaser light source 11 are brought into a relationship of an integernumber times multiple relative each other.

The laser light beam of the laser light source 11 having the frequencyfc of e.g. about 500 to 600 THz is phase-modulated by phase modulator 12with the frequency fm of 10 MHz, such that a side band fc±fm isproduced. An error signal exhibiting polarities is obtained by detectingthe beat between the frequencies of fc and fc±fm of the return lightfrom the external oscillator having the resonant frequency of f₀.

That is, with the electric field E of the fundamental wavelength laserlight source 11 of E₀ exp(i ωc t), the electrical field after themodulation becomes E₀ exp(i (i (ωc t+sin(ωm t))), where ωc is an angularfrequency of the fundamental wavelength laser light, ωm is an angularfrequency of the modulation signal of the phase modulator 12 and β isthe modulation index. If the modulation index is sufficiently small suchthat β<0.2, it suffices to take account of ωc and two sidebands ωc±ωm.Consequently, we obtain the following formula (2)

    E=E.sub.0 [J.sub.0 (β)e.sup.iω.sbsp.c.sup.t +J.sub.1(β) e.sup.i(ω.sbsp.e.sup.+ω.sbsp.in.sup.)t -J.sub.1 (β)e.sup.i(ω.sbsp.c.sup.-ω.sbsp.in.sup.)t ](2)

where J0 (β) and J1(β) are Bessel functions of the first and secondorders, respectively.

Since the complex reflections for ωc and two sidebands ωc±ωm modify therespective terms, the electric field of the reflected light from theexternal resonator 15 becomes

    E=E[J.sub.0 (β)I(Δ.sub.c)e.sup.iω.sbsp.c.sup.t +I.sub.1 (β)I(Δ.sub.c+m)e.sup.i(ω.sbsp.c.sup.+ω.sbsp.m.sup.)t -J.sub.1 (β)I(Δ.sub.c-m)e.sup.i(ω.sbsp.c.sup.-ω.sub.m.sup.)t]

but ##EQU2## Since β<0.2, ##EQU3## and J1 (β) ≈β/2, the followingformula is true (4)

Therefore, if the terms of the second and higher orders of disregarded,the intensity |E|² becomes ##EQU4## but

    A(Δ.sub.c,Δ.sub.c±m)=βE.sub.0.sup.2 Re{I(Δ.sub.c)I.sup.(*) (Δ.sub.c+m)-I(Δ.sub.c)I.sup.(*) (Δ.sub.c-m)}                                        (6)

    B(Δ.sub.c,Δ.sub.c±m)=βE.sub.0.sup.2 Im{I(Δ.sub.c)I.sup.(*) (Δ.sub.c+m)+I(Δ.sub.c)I.sup.(*) (Δ.sub.c-m)}                                        (7)

Synchronous detection of the reflected light with a suitable phase beinggiven to the original modulation signal (with the angular frequencyω_(m)) gives the above formulas (6) and (7) which are the coefficientsof cos (ω_(m) t) and sin(ω_(m) t). The abovementioned error signal maybe obtained from the formula (7) which is the coefficient of sin (ω_(m)t).

That is, FIG. 4 shows a detection signal of the return light (reflectedlight) from resonator 16 in FIG. 1 as detected by the photodetector 23in FIG. 1. This detection signal is a signal component of FIG. 5 as anintensity signal of the reflected light superimposed on a signalcomponent of FIG. 6 corresponding to the modulation signal. Themodulation signal component of FIG. 6 may be taken out by transmissionthrough a band-pass filter having a center transmission frequency of 10MHz which is the above-mentioned modulation signal frequency. If themodulation signal component of FIG. 6 is multiplied by a signal whichaffords a suitable phase to the original modulation signal, andsynchronous detection is performed, the signal component sin (ω_(m) t)as shown in FIG. 7 is obtained. If the signal is freed of the modulationcarrier frequency of 10 MHz by the low-pass filter, the error signalshown by a thick line of FIG. 7, that is the signal of the formula (7),is obtained. Meanwhile, FIG. 8 shows, for reference sake, the signalcomponent of the cos (ω_(m) t) and the signal of formula (6).

FIG. 9 shows, in a perspective view, a typical structure of theelectromagnetic actuator 19 in FIG. 1.

Referring to FIG. 9, the reflective surface 17 of FIG. 1 is formed, suchas by coating, on a reflective mirror 31, which is fitted on aring-shaped or cylindrically-shaped coil bobbin 32 formed of a ceramicor the like insulating material. A coil (so-called voice coil) 33 iswound in the form of a solenoid around the coil bobbin 32. This coilbobbin 32 is mounted on spirally-shaped spring plates 33, as shown inFIG. 10. The spirally-shaped spring plates 34 are secured to andsupported by a ring-shaped yoke 36 via a permanent magnet 35. The magnet35 is mounted for encircling the cylindrically-wound coil 33 of the coilbobbin 32 and is magnetized so that its inner periphery is the N poleand its outer periphery is the S pole. The magnet 35 has its outerperiphery secured to a yoke 36 of iron or the like ferromagneticmaterial. The spring plates 34 are secured, such as by adhesion, to theupper and lower surfaces of the coil bobbin 32. The outer periphery ofeach of the spring plates 34 has its outer periphery supported by theyoke 36. The above-mentioned components are sandwiched between shieldplates 37, 38 of iron or the like ferromagnetic material. These shieldplates 37, 38 also play the part of a return path for the magnetic fluxfrom the magnet 35 in cooperation with the yoke 36. The totality of thecomponents are surrounded by the shield plates 37, 38 for ease ofhandling.

With the electromagnetic actuator, arranged and constructed as shown inFIGS. 9 and 10, the magnetic circuit has a substantially closed magneticpath, despite the fact that an electrically conductive material or amagnetic material is not provided within the coil 33. Characteristicsexhibiting a large thrust (driving force) along the optical axis andless phase deviations may be obtained. On the other hand, the coilbobbin 32 is formed of ceramics to diminish the weight of the movingcomponents, so that the double resonant frequency may be set to 100 kHzor higher.

FIGS. 11 and 12 are Bode diagrams showing transmission characteristicsof a tentatively produced electromagnetic actuator. Specifically, FIGS.11 and 12 show the gain and the phase, respectively. A mirror holder(coil bobbin 32) of the actuator is formed of ceramics, with theresistance, inductance and weight of the actuator being 8 Ω, 570 μH and1.25 g, respectively, and the spring constant and viscosity coefficientof the spring plate 34 being 570 Nm/rad and 0.057 Nm/sec. In thesefigures, resonance is not noticed at 100 KHz and up to nearly 100 KHz off₀. Phase deviations in the higher frequency range are caused by coilinductances.

FIG. 13 is a block diagram of a servo control system. In this figure,initial position or desired position setting signals are supplied at aninput terminal 41 so as to be transmitted to a subtractor 42. An outputsignal from subtractor 42 is servo-phase-compensated at a phasecompensator circuit 43 and converted at a driver 44 into a drivingsignal at a driver 44 which is supplied to the electromagnetic actuator45. The driver 44 and the electromagnetic actuator 45 correspond to thedriver 26 and the electromagnetic actuator 19, respectively, both inFIG. 1. The position of the reflective surface 17 in FIG. 1 of theresonator 15 in FIG. 1 along the optical axis is controlled by theelectromagnetic actuator 45 and a position detection signal for thereflective surface position is transmitted as a subtraction signal to asubtractor 42 where it is subtracted from the desired position signal toproduce a position error signal corresponding to the error signal shownin FIG. 7.

FIG. 14 shows, in a block diagram, a typical arrangement for detectingthe error signal. In this figure, a reflected light detection signalfrom a photodetector 23 shown in FIG. 4 is supplied at an input terminal46 to a low-pass filter (LPF) 47 where it is freed of theabove-mentioned modulation carrier component. An output signal from theLPF 47 is supplied to an additive node 48 where it is added to an offsetDC level from an offset output circuit 49 to produce a reflection signal(reflected light intensity signal) as shown in FIG. 5 so as to be takenout at an output terminal 50.

On the other hand, the reflected light detection signal, supplied to theinput terminal 46, is transmitted through a bandpass filter (BPF) 51where the phase-modulated carrier frequency, such as fm=10 MHz, is takenout and supplied to a sample-and-hold circuit 52 where a processingcomparable to synchronous detection is performed to take out the term ofsin (ω_(m) t) in formula (5). Besides, the modulation carrier componentis removed by the low-pass filter (LPF) so that the component of thecoefficient of sin (ω_(m) t) as shown in FIG. 7 is output at an outputterminal 54. The modulating signal (fm=10 KHz) from the oscillator 21supplied to the input terminal 55 is a waveform shaped by a clockgenerator 56 into pulse signals which are delayed by a predeterminedphase of, for example, 90 degrees, and supplied to the sample-and-holdcircuit 52. The carrier frequency component from the BPF 51 issample-held by the phase-delayed modulation signal to perform asynchronous detection of taking out the above-mentioned sin (ω_(m) t)signal component.

FIG. 15 is a Bode diagram showing closed-loop characteristics of anentire system inclusive of the servo circuit shown in FIG. 13 when theelectromagnetic actuator explained in connection with FIGS. 9 to 12 isemployed. In this figure, curves A and B represent the gain and thephaser respectively. The cut-off frequency may be raised to 20 KHz, byadjusting the gain in the electric circuit. The phase margin at thistime is about 34 meaning that a stable closed loop system may now berealized.

FIG. 16 shows an error signal (A) and a reflected light detection signal(B) when the electromagnetic actuator is driven without servo controlfor deviating the reflecting surface 66 along the optical axis, with apeak-to-peak distance of the error signal (A) being about 1 Å. FIG. 17shows the error signal (A) and the reflecting light detection signal (B)when the closed loop servo are applied. It is seen that fluctuations ofthe error signal (A) is suppressed to not more than ±0.1 Å while thereflected light detection signal (B) is approximately zero so thatsubstantially all of the laser light beam has been introduced into theexternal resonator 15.

FIG. 18 shows a modification of a laser light emitting apparatusaccording to the present invention, in which the laser light beam of thefundamental wavelength, radiated from a laser light source 61, isphase-modulated by a phase modulator 62 so as to be incident via a lightconverging lens 64 to an external resonator 65. The external resonator65 is made up of a reflective surface 66 of a concave mirror, areflective surface 67 of a concave mirror 67, and a non-linear opticalcrystal element 68 arranged therebetween, so that an optical path of aresonator 65 is defined by these reflective surfaces 66, 67 and thereflective surface 63 of the plane mirror. The resonator is operated inresonance when the optical path length L_(R) of the resonator 65 ischanged such that the optical path phase difference becomes equal to aninteger number times 2π so that the reflection and the reflection phaseare changed acutely. The reflective surface 66 of the resonator 65 isdriven along the optical path by the electromagnetic actuator 69.

The arrangement from the oscillator 21 to the driver 26 is the same asthat of the embodiment shown in FIG. 1, so that description is omittedfor brevity. The electromagnetic actuator 69 may be arranged andconstructed as shown in FIGS. 9 and 10. The operation of the variouscomponents is similar to that of the above-described embodiment andhence the description is again omitted for brevity.

The laser light beam generating apparatus according to the presentinvention may be designed in many ways other than in the above-describedembodiments. Several basic arrangements of the laser light generatingapparatus according to the present invention are hereinafter explainedby referring to FIGS. 19 to 23.

FIG. 19 shows a first basic arrangement of the present invention inwhich a so-called SHG laser resonator as a solid-state laser resonatoris employed as a laser light source 11 shown in FIG. 1. Referring toFIG. 19, a resonator 91 for SHG laser light beam generation includes alaser medium 94, such as Nd:YAG, and a non-linear optical crystalelement 95, such as KTP (KTiOPO₄), arrayed between a pair of reflectingsurfaces 92, 93. An excitation light beam, radiated from an excitationlight source, such as a semiconductor laser 101, is converged via alight converging lens 102 on the laser medium 94 of the resonator 91.The laser light beam having the fundamental wavelength of 1064 nm, forexample, is radiated from the laser medium 94 and transmitted throughthe non-linear optical crystal element 95 for resonation within theresonator 91 for generating the SHG laser light beam of the wavelengthof 532 nm. The SHG laser light beam is phase-modulated by a phasemodulator 12 shown in FIG. 1 and caused to be incident via a reflectingsurface 13 for detecting the reflected light beam from the resonator andvia the light converging lens 14 into an external resonator 75. One ofthe reflecting surfaces 76, 77 of the external resonator 75, forexample, the reflecting surface 76, is driven along the optical axis ina controlled manner by the electromagnetic actuator 79. Within theexternal actuator 75, a laser light beam having the wavelength of 266nm, which is the second harmonic of the incident laser light beam, thatis the fourth harmonic of the original laser light beam with thewavelength of 1064 nm, is generated and taken out of the externalresonator 75. The arrangement of the oscillator 21, the driver 22, thephotodetector 23, the synchronous detection circuit 24, the low-passfilter (LPF) 25 and the driver 26 is the same as the above-describedfirst embodiment and hence the explanation is omitted for simplicity.

FIG. 20 shows a second basic arrangement of the present invention inwhich a solid-state laser resonator having a pair of reflective surfaces72, 73 and a laser medium 74, of such as Nd:YAG etc arrangedtherebetween is employed as the above-mentioned laser light source. Inthis resonator, the laser light beam of the fundamental wavelength of1064 nm, for example, is introduced from the laser light source througha non-linear optical crystal element 78, such as lithium niobate(LiNbO₃) arranged between the reflective surfaces 76, 77 of the externalresonator 75 for generating second harmonics having the wavelength of532 nm. One of the reflective surfaces of the external resonator 75,such as the reflective surface 76, is position-controlled along theoptical axis by the above-mentioned electromagnetic actuator 79.

FIG. 21 shows a third basic arrangement of the present invention inwhich a solid-state laser resonator having a pair of reflective surfaces82, 83 and a laser medium 84 of such as Nd:YAG etc arranged therebetweenis employed as the above-mentioned laser light source, and in which thelaser light beam of the fundamental wavelength of 1064 nm, for example,is introduced from the laser light source through a non-linear opticalcrystal element 88, such as lithium niobate (LiNbO₃) arranged betweenthe reflective surfaces 86, 87 of the external resonator 85 forgenerating second harmonics having the wavelength of 532 nm. One of thereflective surfaces of the resonator 81, such as the reflective surface83, is position-controlled along the optical axis by the above-mentionedelectromagnetic actuator 89. With the present third basic arrangement,reflection of the laser light beam with respect to the externalresonator 85 is changed by the oscillation frequency of the laser lightbeam of the fundamental laser light beam from the laser light sourcebeing changed, thereby establishing a stable state in which laser lightbeam introduction into the external resonator 85 is increased.

In these basic arrangements, shown in FIGS. 20 and 21, Nd:YVO₄, LNP,Nd:BEL, etc. may be used as the laser media 74, 84,in addition toNd:YAG. The non-linear optical crystal elements 78, 88 may also be KTP,QPM LN, LBO or BBO besides LN.

Although not shown, one of the reflective mirrors of the SHG laserresonator as a laser light source may be driven by the electromagneticactuator as in the case of the above-mentioned first basic arrangement.If the second harmonic generating type laser resonator generating thesecond harmonic laser light beam within the resonator is employed as alaser light source, and the laser oscillator is of the homogeneous linebroadening as is the solid-state laser resonator, an oscillation of thepolarization of the mode closest to the peak of the gain curve (gainfrequency characteristic curve) is produced and the gain is saturated sothat the single mode oscillation is produced. However, in effect,multi-mode oscillation is produced due to the hole burning effects. Thisis because the standing wave is present within the laser resonator 13and the gain is not fully saturated at the node of the standing wave, asa result of which oscillations having a different mode are produced.Should longitudinal multi-mode be present in the same polarization modeof the laser light beam of the fundamental wavelength, there is a riskthat the mode hop noise due to mode coupling in one and the samepolarization mode tends to be produced within the same polarizationmode.

In the specification and drawings of Japanese Patent ApplicationNo.2-125854, the present Assignee has proposed arranging an opticaldevice inhibiting coupling of two polarization modes of the laser lightbeam of the fundamental wavelength due to generation of sum frequency,or a so-called etalon, within the laser resonator, or arranging thelaser medium 16 in proximity to the quarter wave plate 15, forinhibiting the multi-mode oscillation due to the above-mentionedhole-burning effect. In the specification and drawing of the JapanesePatent Application No.3-17068, the present Assignee has also proposedproviding an optical element inhibiting coupling of the two intrinsicpolarization modes of the laser light of the fundamental wavelength, andan adjustment device or adjusting polarization so that the laser lightbeam of the fundamental wavelength propagated back and forth in thelaser medium will become circular polarization. It is preferred toinhibit hole burning effects in the SHG laser resonator or to preventthe mode hop noise from being produced by the techniques disclosed inthese Publications.

By setting the optical path length of the SHG laser light source so asto be an integer number times as large as the optical path length of theexternal resonator, the SHG laser light beam can be introducedefficiently into the external oscillator. This arrangement is requiredin order that the longitudinal modes of the SHG laser light beam, whichare based on the two intrinsic polarization modes of the fundamentalwavelength laser light beam produced by introducing a double refractiondevice such as a quarter wave plate in the resonator of the SHG laserlight source adapted for establishing the so-called type II phasematching conditions between the fundamental wave laser light beam andthe SHG laser light beam, will be introduced in their entirety into theexternal resonator. The multi-modes may be efficiently introduced bysetting the optical path length of the SHG laser light source so as tobe an integer number times that of the external resonator.

That is, by introducing the SHG laser light beam from the SHG laserresonator into the external resonator having an internal non-linearoptical crystal element, in which the frequency difference of thelongitudinal modes within the two polarization modes of the resonatorhaving the internal non-linear optical crystal element is equal to anodd number multiple of one half the interval of the longitudinalresonance modes, and by setting the optical path length of the externalresonator so as to be an integer number times the optical path length ofthe SHG laser resonator, two or more modes of the laser light beam fromthe SHG laser resonator may be simultaneously introduced into theexternal resonator to improve the multi-stage wavelength conversionefficiency.

FIG. 22 shows a fourth basic arrangement of the solid-state laserresonator according to the present invention in which two externalresonators 75a, 75b are arranged in series with each other. In theembodiment shown in FIG. 22, a laser light beam from the resonator 71 ofthe fundamental wave laser light beam having the wavelength of e.g. 1064nm is introduced into a first external resonator 75a for converting thelaser light beam into the SHG laser light beam having the wavelength of532 nm by the non-linear optical crystal element 78a such as LiNbO₃. TheSHG laser light beam thus produced is introduced into a second externalresonator 75b for converting the SHG laser light beam into the laserlight beam of the fourth harmonic of 266 nm wavelength (FHG) by thenon-linear optical effects of the non-linear optical crystal element78b, such as BBO. One of the reflective surfaces 76a, 77a of the firstexternal resonator 75a, such as the reflective surface 76a, is shiftedalong its optical axis by the electromagnetic actuator 79a, while one ofthe reflective surfaces 76b, 77b of the second external resonator 75b,such as the reflective surface 76b, is shifted along its optical axis bythe electromagnetic actuator 79b, until the conditions concerning theoptical paths of the resonators 71, 75a and 75b are satisfied.

FIG. 23 shows a fifth basic arrangement of the present invention inwhich wavelength conversion is performed by so-called sum frequencymixing. That is, the SHG laser light beam of the wavelength of 532 nmfrom a laser resonator 91 as the aforementioned SHG laser light sourceas explained with reference to FIG. 19 is transmitted via a wavecombining mirror 97, such as a dichroic mirror, to an external resonator85. One of reflecting surfaces 92, 93 of the resonator 91 of the SHGlaser light source, for example, the reflecting surface 93, is shiftedalong the optical axis by an electromagnetic actuator 96 such as theabove-described electromagnetic actuators. The laser light beam from alaser resonator 81 as shown in FIG. 21 is transmitted to an externalresonator 85 via a wave combining mirror 97 after deflection by a mirror(reflective surface) 98. In the external resonator 85, the laser lightbeam of 532 nm wavelength and the laser light beam of the 1064 nmwavelength are sum frequency mixed by the non-linear optical effect ofthe non-linear optical crystal element 88, such as an MMO element, forproducing a laser light beam of, for example, the wavelength of 355 nm,which is outputted.

The present invention is not limited to the above-described embodiments.For example, the wavelength of the fundamental laser light from thelaser medium of Nd:YAG may be 956 nm or 1318 nm, besides 1064 nm. Thelaser light source may also be a semiconductor laser, such as a laserdiode, or a gas laser, such as He-Ne laser, besides the solid-statelaser. The laser light beam from the light sources for sum frequencymixing as shown in FIG. 23 may also be the laser light beam from theexternal resonator as shown in FIG. 22.

What is claimed is:
 1. A laser light beam generating apparatuscomprising:at least one light beam source for emitting a light beam; afirst reflector; a second reflector; a non-linear optical crystalelement provided between said first and second reflectors, a light beamfrom said light beam source being incident on said non-linear opticalcrystal element through said first reflector; and actuating means foractuating at least one of said first and second reflectors along anoptical axis of the light beam emitted from said light beam source,wherein said actuating means comprises a reflector supporting member forsupporting said one reflector and an electromagnetic actuator for movingsaid one reflector supported by said supporting member and wherein saidelectromagnet actuator includes: a ring shaped magnet surrounding saidreflector supporting member; a spring plate connected to said magnet;and a shield plate connected to said spring plate, said shield platebeing formed from a ferromagnetic material.
 2. An apparatus as recitedin claim 1 further including a yoke surrounding said magnet, said yokebeing formed from a ferromagnetic material.
 3. A laser light beamgenerating apparatus comprising:at least one light beam source foremitting a pumping light beam; a first resonator including first andsecond reflectors and a laser medium into which the pumping light beamis incident from said light beam source through said first reflector; asecond resonator including third and fourth reflectors and a non-linearoptical crystal element in which the light beam from the first resonatoris incident through said third reflector; and actuating means foractuating at least one of said first, second, third and fourthreflectors along an optical axis of the light beam emitted from saidfirst resonator, wherein said actuating means comprises a reflectorsupporting member for supporting said one reflector and anelectromagnetic actuator for moving said one reflector supported by saidsupporting member, said electromagnet actuator includes: a ring shapedmagnet surrounding said reflector supporting member; a spring plateconnected to said magnet; and a shield plate connected to said springplate, said shield plate being formed from a ferromagnetic material. 4.An apparatus as recited in claim 3 further including a yoke surroundingsaid magnet, said yoke being formed from a ferromagnetic material.